Compasses

Compasses

Introduction
Once the basic skills of map interpretation have been mastered, it is time to work on the skill of compass reading.

The compass is the basic direction-finding device in terrestrial navigation. There are three categories of compasses: magnetic compasses depend on the earth's magnetic field, gyrocompasses rely on the rotation of the earth, and solar compasses use the location of the sun and stars. For our purposes, the magnetic compass is the simplest to use, and further discussion will be confined to this type of instrument.

Magnetic compass
There are several types of magnetic compasses. Some have special uses and others are for general use.

For serious map and compass work, the best type of compass is an Orienteering compass (see diagram). This compass is especially designed for use with topographic maps and is inexpensive and durable.

Basic principle of a compass The magnetic compass works on the principle that the earth is a giant magnet and generates a magnetic field running north and south exactly like a bar magnet. A freely suspended iron needle will align itself with the lines of magnetic force of the earth. There are several difficulties with the use of a magnetic compass. First, the earth's magnetic field does not coincide exactly with its polar axis. Thus the magnetic north pole and the true North Pole are some distance apart. As you will learn in this unit, however, it is quite easy to correct the error caused by this discrepancy. If you are close to the North Pole, however, the compass needle tends to point south. For general use in our area, this is not a major problem. Finally, metal objects, such as belt buckles, boats, automobiles, or even large iron ore deposits in the earth, can cause a compass to give misleading information. The metal interferes with the compass needle's response to the magnetic north pole. These are minor problems and can be easily corrected if you are aware of a compass' limitations.

Parts of an Orienteering compass
The Orienteering compass consists of three basic parts:

magnetic needle,
a revolving compass housing,
and a transparent base plate.

Each has its own special function. Together they give you instant directions, combining compass, protractor, and ruler into a single tool.

Etched on the bottom of the housing is the orienting arrow. Around the housing margin is the scale graduated in 360 degrees of a circle (north is at 0 degrees and south is at 180 degrees). The compass housing can be rotated on the base plate independent from the needle. Inscribed on the base plate, in line with the direction-of-travel arrow, is the index mark. The number appearing over or adjacent to the index mark is called the compass bearing.

Finding direction using a compass
Finding direction with an Orienteering compass is easy. To locate a specific direction-for example, north-rotate the compass housing so 0° (north) is directly over the index mark. Hold the compass at your waist. Be careful of nearby metal objects, such as belt buckles, automobiles, or metal buildings which might influence the needle. Turn your body holding the compass steady until the red end of the floating needle is aligned with the orienting arrow etched on the bottom of the compass housing. Look up-the direction you are facing is north. This procedure is called "setting a bearing."

To determine the direction of a distant object with an Orienteering compass, face the direction in question. Hold the compass either waist or chest high with the direction-of travel arrow pointing straight ahead and the protractor plate level. Now rotate the compass housing until the floating needle is aligned over the orienting arrow etched on the bottom of the compass housing. The north end of the needle should point toward the tip of the orienting arrow. Read the bearing, in degrees, from the scale at the index mark. This procedure is called "taking a bearing."

Finding direction from a map
Finding direction from a map is just as simple. First note that the top of the topographic map is true north. Select a point on the map to serve as a base or reference point-a road junction, for example. Select a second point, such as a pond, to serve as a destination. An arrow drawn from the reference point to the destination point indicates the direction of travel. Place the compass on the map so that the base plate lies along the direction-of-travel line you have drawn on the map. The direction-of-travel arrow on the compass should also point in this direction. With the compass in this position, rotate the compass housing until the orienting arrow etched on the bottom of the compass points north (top of the map). Read the bearing on the compass dial at the index mark. This compass bearing is then the direction you should follow from the road junction to the pond.

Note that the floating needle is ignored throughout this procedure. Also note that the bearing arrived at is the true bearing. It probably will not be the same bearing obtained from actual measurements in the field. The next section explains why.

Right: Hold the compass at waist level with the direction of travel arrow pointing straight ahead, then rotate the compass housing until the floating needle (red end) is aligned over the orienting arrow. Keep the compass level.

Introduction	Once the basic skills of map interpretation have been mastered, it is time to work on the skill of compass reading. The compass is the basic direction-finding device in terrestrial navigation. There are three categories of compasses: magnetic compasses depend on the earth's magnetic field, gyrocompasses rely on the rotation of the earth, and solar compasses use the location of the sun and stars. For our purposes, the magnetic compass is the simplest to use, and further discussion will be confined to this type of instrument.
Magnetic compass	There are several types of magnetic compasses. Some have special uses and others are for general use. For serious map and compass work, the best type of compass is an Orienteering compass (see diagram). This compass is especially designed for use with topographic maps and is inexpensive and durable.
Basic principle of a compass	The magnetic compass works on the principle that the earth is a giant magnet and generates a magnetic field running north and south exactly like a bar magnet. A freely suspended iron needle will align itself with the lines of magnetic force of the earth. There are several difficulties with the use of a magnetic compass. First, the earth's magnetic field does not coincide exactly with its polar axis. Thus the magnetic north pole and the true North Pole are some distance apart. As you will learn in this unit, however, it is quite easy to correct the error caused by this discrepancy. If you are close to the North Pole, however, the compass needle tends to point south. For general use in our area, this is not a major problem. Finally, metal objects, such as belt buckles, boats, automobiles, or even large iron ore deposits in the earth, can cause a compass to give misleading information. The metal interferes with the compass needle's response to the magnetic north pole. These are minor problems and can be easily corrected if you are aware of a compass' limitations.
Parts of an Orienteering compass	The Orienteering compass consists of three basic parts: magnetic needle, a revolving compass housing, and a transparent base plate. Each has its own special function. Together they give you instant directions, combining compass, protractor, and ruler into a single tool. Etched on the bottom of the housing is the orienting arrow. Around the housing margin is the scale graduated in 360 degrees of a circle (north is at 0 degrees and south is at 180 degrees). The compass housing can be rotated on the base plate independent from the needle. Inscribed on the base plate, in line with the direction-of-travel arrow, is the index mark. The number appearing over or adjacent to the index mark is called the compass bearing.
Finding direction using a compass	Finding direction with an Orienteering compass is easy. To locate a specific direction-for example, north-rotate the compass housing so 0° (north) is directly over the index mark. Hold the compass at your waist. Be careful of nearby metal objects, such as belt buckles, automobiles, or metal buildings which might influence the needle. Turn your body holding the compass steady until the red end of the floating needle is aligned with the orienting arrow etched on the bottom of the compass housing. Look up-the direction you are facing is north. This procedure is called "setting a bearing." To determine the direction of a distant object with an Orienteering compass, face the direction in question. Hold the compass either waist or chest high with the direction-of travel arrow pointing straight ahead and the protractor plate level. Now rotate the compass housing until the floating needle is aligned over the orienting arrow etched on the bottom of the compass housing. The north end of the needle should point toward the tip of the orienting arrow. Read the bearing, in degrees, from the scale at the index mark. This procedure is called "taking a bearing."
Finding direction from a map	Finding direction from a map is just as simple. First note that the top of the topographic map is true north. Select a point on the map to serve as a base or reference point-a road junction, for example. Select a second point, such as a pond, to serve as a destination. An arrow drawn from the reference point to the destination point indicates the direction of travel. Place the compass on the map so that the base plate lies along the direction-of-travel line you have drawn on the map. The direction-of-travel arrow on the compass should also point in this direction. With the compass in this position, rotate the compass housing until the orienting arrow etched on the bottom of the compass points north (top of the map). Read the bearing on the compass dial at the index mark. This compass bearing is then the direction you should follow from the road junction to the pond. Note that the floating needle is ignored throughout this procedure. Also note that the bearing arrived at is the true bearing. It probably will not be the same bearing obtained from actual measurements in the field. The next section explains why. Right: Hold the compass at waist level with the direction of travel arrow pointing straight ahead, then rotate the compass housing until the floating needle (red end) is aligned over the orienting arrow. Keep the compass level.